Roller screws are used to convert rotational movements into linear displacements, and vice versa. The rolling elements are threaded rollers arranged between a screw and a nut. The rollers are spaced apart and are contained within a cylindrical ring around the screw. They are also called “satellite roller screws”.
The large number of points of contact generally enables satellite roller screws to support very high loads.
Roller screws are currently preferred to ball screws in some applications. Notably, they have an advantage over ball screws when the acceptable static and dynamic loading capacities are higher.
The threaded rollers perform the rolling function instead of the balls, and the load is distributed over a greater number of points of contact.
The roller screws may also have pitches corresponding to whole numbers or to real numbers, which is advantageous for estimating the reduction in force and calculating the distances of translational movement. The pitch may be chosen freely and made without any particular modification to the geometry of the nut or the screw.
Several types of roller screws exist. Notably, there are non-recirculating roller screws, recirculating roller screws and other arrangements such as inverted roller screws.
A roller screw with rollers that do not recirculate comprises a nut which has an internal thread that is identical to that of the screw.
The rollers have a single-start thread with a helix angle corresponding to that of the nut. There is thus no axial displacement between the nut and the rollers. There is therefore no need for the rollers to recirculate.
A recirculating roller screw comprises a thread. The nut has a thread that is identical to the thread of the screw. The rollers are not threaded but have grooves arranged perpendicularly to the axis of the screw. The distance between the grooves corresponds to the transverse pitch of the screw and the nut.
When the screw or the nut rotates, the rollers are displaced axially in the nut. After one complete revolution, each roller is returned to the starting position by two cams fixed at the ends of the nut. This recirculating of the rollers is made possible by a longitudinal groove in the nut.
FIG. 1 shows an embodiment of such a roller screw. A roller screw 100 comprises a plurality of rollers 102 contained within a structure 103 which allows the loads to be distributed satisfactorily whilst preserving a spacing between the rollers and ensuring contact, on the one hand, between the rollers 102 and the screw 100 and, on the other hand, between the rollers 102 and the nut 101 which has an internal thread. The unit formed by the structure 103, the rollers 102 and the nut 101 is held between two cams 104 which enable the rollers to return to their starting position after one complete revolution.
An inverted roller screw has a thread. The nut has a thread that is identical to the thread of the screw. The kinematic movement is inverted as in this case it is the nut which moves in translation under the rotational effect of the screw.
The reduction capacities of the screws are today limited by the form of the pitches. By way of example, 0.5 mm is the accuracy of translational movement achieved by screws from the prior art.
Moreover, the need for a high reduction is incompatible with a high load absorption capacity, as the threads have a small cross-section. This incompatibility necessitates a much larger screw diameter, a greater roller length and a higher number of rollers. Thus for a rolling bearing offering a high reduction and a high load absorption capacity, the rolling screw becomes too large.